Vibrating meters, such as for example, liquid density meters, gas density meters, liquid viscosity meters, gas/liquid specific gravity meters, gas/liquid relative density meters, and gas molecular weight meters, are generally known and are used for measuring characteristics of fluids. Generally, meters comprise a sensor assembly and an electronics portion. The material within the sensor assembly may be flowing or stationary. Each type of sensor may have unique characteristics, which a meter must account for in order to achieve optimum performance. For example, some sensors may require a tube apparatus to vibrate at particular displacement levels. Other sensor assembly types may require special compensation algorithms.
The meter electronics, among performing other functions, typically include stored sensor calibration values for the particular sensor being used. For example the meter electronics may include a reference sensor time period (i.e. the inverse of the reference resonant frequency). The reference sensor time period represents a fundamental measurement performance of the sensor geometry for a specific sensor assembly, as measured in the factory under reference conditions. A change between a sensor time period measured after a vibrating element meter is installed at a customer site and a reference sensor time period may represent physical change in the sensor assembly due to coating, erosion, corrosion, or damage to the vibrating element sensor, in addition to other causes.
A commonly used technique to monitor a change of sensor time period in vibratory meters is to perform an air-point health check, a vacuum-point health check, or a health check using any fluid having an accurately known density. In any of the three health check methodologies, a meter is taken off-line and placed under test conditions. The meter is sometimes cleaned before being placed under test conditions, either through mechanical or solvent-based techniques. Either a liquid or gas meter may next be placed under a vacuum or filled with a fluid having an accurately known density, such as air or water. For a liquid meter, the test conditions commonly include placing the meter under ambient air conditions. For a gas meter, the test conditions commonly include placing the meter under vacuum conditions. The sensor time period is then determined and compared to the reference sensor time period measurement.
Typically, test measurements are taken under conditions that may be different from the reference conditions of a health check test. The sensor time period measured during a health check may therefore reflect variations in vibrational response due not only to changes in a sensor assembly, but also due to differences between reference and test conditions. Current health check methodologies fail to isolate changes in vibrational response due to changes in the physical sensor assembly and changes in test conditions.
For example, the sensor time period measurement may be affected by temperature. The first reason that temperature may affect a sensor time period is because temperature may affect the stiffness of the sensor assembly itself. The second reason is because the density of fluid moving in a sensor assembly may be dependent on temperature. A third mechanism that temperature may affect the robustness of a health check is if the sensor assembly is not at a stable temperature or if there is a temperature drift. None of these temperature effects are accounted for under the conventional vibratory sensor health check techniques, which may lead to false indications that a sensor assembly is either faulty or healthy. Errors may lead to incorrect customer decisions and unnecessary service calls.
What is needed is a sensor health assessment that corrects for variations in measured sensor time period due to temperature, pressure, and density. What is also needed is a method to determine whether a sensor assembly is stable enough to provide an accurate result from an air-point health check, a vacuum-point health check, or a health check using another fluid.